It has been known that minute modified regions which have a different refractive index from that of the vicinity thereof are formed when a short pulse laser is condensed on the inside of a transparent medium such as silica glass. It has been discovered that a large number of the minute modified regions are three-dimensionally formed inside the medium to be used for recording information.
The following PTLs 1 and 2 and NPL 1 are literature relating to such a recording medium. A mechanism in which local modification is caused by the short pulse laser is, for example, disclosed in NPL 2.